Capacitive pickups for digitizers have been in use for some time. Perhaps the simplest of these devices are the hand held stylii, which resemble, and often incorporate, ball point pens. In this class of device the essential element is a conductive region, at or near the tip, having capacitive coupling to elements in the digitizing surface, or platen. A stylus suffers from sveral drawbacks. First, it tends to be sensitive to error induced due to the operator's inadvertent tilting of the stylus. Second, it is sometimes difficult to adequately shield the stylus from the effects of stray capacitive coupling, e.g., to the hand of the operator. And last, the small tip of a stylus does not afford much coupling between the pickup and the platen, particularly in comparison to other capacitive pickup techniques. In consequence, stylii are used in applications involving low to medium accuracy where the convenience of a stylus, and possibly the utility of writing with the stylus, are important.
Increased accuracy of positioning and freedom from tilt error are obtained by use of a cursor having a reticle, or cross hairs. Such cursors have a broad flat surface that automatically prevents tilting when the cursor is placed on the platen. The cross hairs allow positioning with great accuracy, often to within a few thousandths of an inch. Transparency is a virtual requirement in this class of capacitive pickups, particularly in the immediate region of the cross hairs. However, this can give rise to some significant construction problems.
A transparent conductive disc could be used as the pickup. It would provide good coupling and would not interfere with the operator's view of the workpiece. It could be deposited on a single transparent substrate such as glass, and centered with the cross hairs. However, the conductors going to the disc should be shielded, and that implies costly multi-layer techniques.
Another solution has been to support, in the interior of the reticle, a circular metal ring whose center is coincident with the intersection of the cross hairs. Shielding and alignment are potential serious problems with this approach.
Yet another solution has been to surround the exterior of the reticle with a conductive circular ring, or with a circular pattern of symmetrically placed discs. The last two techniques also provide more coupling than a stylus does, but since the effective center of the pickup is now further from the actual region of coupling, alignment errors can be difficult to control. In addition, if a series of discs are used, rather than a continuous ring or actual disc, and if the digitizing technique is sensitive to changes in the amplitude of the sensed signal, errors may arise due to varying degrees of coupling from the discrete discs to discrete elements in the platen as the cursor is rotated about its cross hairs.
An additional disadvantage of any scheme utilizing coupling to conductors exterior to the reticle is that effective diameter of the pickup is made larger than the diameter of the viewing area. Thus, if the user positions the intersection of the cross hairs near the edge of the active area of the platen he may inadvertently cause a portion of the actual pickup to extend beyond the active area, with a resultant drastic loss of coupling. To counter this, systems using such pickups must mark the limits of the usable digitizing surface as a more diminished region of the actual active area than would be necessary if the diameter of the pickup were smaller.
Each of these difficulties is either diminished or absent in an aperture pickup utilizing transparent conductors. An aperture pickup affords good coupling because it essentially presents a solid disc of substantial size. Rotational effects are absent because the effective pickup disc is easily made circular by use of a circular aperture. Assembly is simple, requiring only precise alignment of the aperture with respect to the cross hairs, and these can easily be on opposite sides of the same piece of glass. The aperture pickup is easily shielded by including in the reticle an additional piece of glass entirely covered on one side with a transparent conductor. Connections to the various conductive coatings are easily made by flexible metal fingers if the glass pieces possess different sizes or different shapes, so that each layer of transparent conductor is partially exposed when the glass pieces are laminated together. In addition, because an aperture pickup affords good coupling, the diameter of the aperture can be small--so that it is well within the viewing area of the reticle. This allows the usable region of the digitizing surface to be a larger portion of the actual active digitizing surface than is possible with pickup schemes whose actual region of coupling is exterior to the reticle.